Weakening Process Due To Phyllosilicate Dehydration And Dehydroxylation Reactions And Its Possibility To Trigger Earthquakes

We experimentally simulated the dewatering reactions from the phylloscilicate minerals to weaken the fault due to the abnormal pore pressure generation, and discussed those possibilities to trigger earthquakes. For the experiment, the gouge samples, Na-montmorillonite, kaolinite and antigorite were subjected to sliding with heating at 9.6° C min-1 to 500° C and 800° C under 80 MPa of confining pressure. Significant frictional-strength reductions were found in the Na-montmorillonite gouge due to dehydration and in the kaolinite and antigorite gouges due to dehydroxylation when we controlled the drainage condition surrounding the gouge zone as to be "undrained". However, frictional strength continued to increase in the fully drained condition that allowed released water to move outside the gouge. Frictional strengths under the two drainage conditions were compared during dehydration and dehydroxylation reactions to estimate the pore pressure generated in the gouge zone under the undrained condition. The 6-8 wt.% of dehydroxylated water generated from the kaolinite caused a rapid increase in pore pressure to 68 MPa at 500° C. The dehydroxylation of antigorite induced weakening gradually from 500° C prior to the rapid reduction appeared at 620° C, corresponding to ca. 70 MPa of pore pressure. In contrast, in the Na-montmorillonite gouge, the pore pressure gradually increased to 72 MPa as the temperature rose to 500° C. Especially on a coseismic stage, these results suggest that dehydration/dehydroxylation could advance the frictional sliding acceleration induced by frictional heating, even if the fault is in a dry condition so long as the fault rock contains the minerals having water in their crystals, such as phyllosilicates.